Handheld pipettes are commonly used to dispense or transfer small but accurately measured quantities of liquids.
Air displacement pipettes are the most common variety of handheld pipettes. In an air displacement pipette, a controllable piston is mounted for movement axially within a chamber in the pipette; the piston moves in response to either manual or motorized electronic control. Typically, the piston moves in a chamber in the liquid end, or shaft, of the pipette, to which disposable pipette tips may be mounted.
An air tight seal is formed between the piston and the shaft. With such a seal in place, axial movement of the piston will vary the size of the airspace within the shaft. Moving the piston downward, into the shaft, will reduce the airspace and force air out of the shaft through an open distal end. Moving the piston upward, out of the shaft, will increase the airspace and cause air to be drawn into the shaft through the open end. The seal between the piston and the shaft is generally formed with a compressed O-ring or a similar structure, fabricated from a material that provides satisfactory long-term performance. For example, a piston seal structure may be made from polyethylene combined with PTFE, which has been found to offer good sealing performance and wear resistance and reliability over a period of months, although such seals do tend to break down and leak over the course of time. Other configurations are possible, including various dry or lubricated seals.
A disposable pipette tip is then sealed to the open distal end of the shaft. Then, as the piston is moved within the shaft, air—or a measured quantity of liquid equal in volume to the displaced air—is drawn into or forced out of the tip. With both the piston and the tip sealed to the shaft, the only entry and exit path should be the distal open end of the disposable pipette tip. Because of the sealed system, air displacement pipette may be used to make accurate and precise measurements, and to move carefully calibrated quantities of liquids.
In general, disposable tips attach to the shaft of a pipette through a simple friction fit. The two most common and commercially successful pipette tip fitting standards are the standard conical mount and the LTS® system offered by Rainin Instrument, LLC. In both cases, the friction fit between the shaft and the disposable tip also enables an air-tight seal. One version of the LTS pipette tip is described in U.S. Pat. No. 6,168,761, which is hereby incorporated by reference as though set forth in full. The LTS tip, as shown, seals against the pipette shaft along a single, thin, annular sealing band, and the commercial success of the LTS system shows that successful and repeatable sealing, with low friction, may be accomplished with such a configuration.
Other tip mounts have been attempted but have not met with great commercial success. Matrix Technologies Corporation offers a tip with tabs that lock into corresponding features in the pipette shaft. See U.S. Pat. No. 7,641,859 to Cote et al. Viaflo Corporation has a solution that includes a lobed shaft that locks into corresponding features within the tip. See U.S. Pat. No. 7,662,343 to Mathus et al. Sorenson BioScience sells a dual-material pipette tip for traditional pipette shafts that employs a second material to optimize the mount, but still seal adequately. These dual-material tips have a rigid polypropylene distal end for handling liquids, and a soft thermoplastic elastomeric mount portion overmolded thereon. See U.S. Patent Application Publication No. 2008/0078258 to Price et al. Tips for robotic applications from Apricot Designs have external seal rings that seal to the inner barrel of a tip holder. See U.S. Pat. No. 6,780,381.
All of these alternative pipette tips share a common attribute: not only do they require a seal between the tip and the shaft, but they also require a seal between the shaft and the piston. There are two seals, and two potential points of failure. The seal between the tip and the shaft is replaced every time a tip is discarded and replaced with a new one, but the seal in the pipette is serviced infrequently. This may lead to leaks and other failures, which in turn may lead to inaccuracy in liquid measurement or failure in pipetting operations.
In general, seal failure (such as wear, splitting, other damage, misalignment, dislodgment, corrosion, or contamination) is a common cause of pipetting failure. These failures can lead to failed outcomes, and may be difficult to identify in advance, or even as pipetting is ongoing. Wear and damage to the shaft in the tip mount region can also result in failures, and for this reason, plastic pipette shafts are also replaced from time to time.
These problems may be mitigated to some extent by performing frequent calibrations and having pipette serviced relatively often. Best practices in this regard frequently involve regular seal replacement, even if it does not appear necessary. Such maintenance often involves fairly significant teardown of the pipette, which requires dedicated labor and calibration upon reassembly. It has the potential to take a pipette out of service for a period of time.
Another consequence of the traditional pipette configuration—with the piston moving within a cavity in the shaft, which is in turn connected to a disposable tip—is the existence of significant empty ullage space over the liquid. A substantial cushion of air exists between the piston and the liquid level in the tip. This cushion of air is capable of expansion and contraction when acted upon, serving as a flexible “spring” between the position of the piston and the liquid level. This additional movement is undesirable, and may lead to volume inaccuracies. Moreover, the cushion of air is potentially subject to liquid evaporation into the air, heating, and cooling, and resulting expansion and contraction effects, which may further affect the accuracy of pipetting operations. High accuracy is still possible, but it is largely dependent on appropriate technique being employed by the user.
In traditional pipette tips, protection from cross-contamination is generally accomplished by inserting a disc or cylinder of porous filter media near the proximal end, between the mount of the pipette tip and the liquid-handling portion. The filter allows air to pass through, but inhibits aerosols and liquids. Such filtered tips must be larger than unfiltered tips for the same liquid volume capacity (because of the space occupied by the filter, plus a gap between the filter and the liquid level). Filters also tend to impede airflow, and are relatively inefficient and expensive to produce and insert into pipette tips. Because of this, filtered pipette tips are generally more expensive than their unfiltered counterparts.